Water jet propulsion unit for use in water borne craft

ABSTRACT

A water jet propulsion unit for water borne craft including two contra rotating impellers, located in an upstream pump housing and a downstream pump housing respectively, and mounted on coaxial shafts. A pressure control priming device in the form of a spring loaded collapsible skirt is located between the impellers. The pressure in the downstream pump housing can be maintained at atmospheric pressure by the controlled admission of air through air inlets.

TECHNICAL FIELD

The present invention relates to water jet propulsion units for use inwater borne craft

BACKGROUND OF THE INVENTION

This specification describes three water jet propulsion unit designswhich contain a pair of counter-rotating impellers in in-linearrangement being driven forwardly on two coaxially arranged shafts. Themeans for driving the impellers are typically described in our NZ PatentNo. 256488.

The designs depart from previous design and operating criteria, in thatwe require that the downstream impeller, in each case, operate atatmospheric pressure. Unlike the designs described in our NZ patent No256488, where a hydraulic balance is maintained so that nozzle/internalpump pressures are in the range of about 0 to 276 kPa, in these designsonly the upstream impeller/nozzle section operates within this pressureregime. Notwithstanding this, the upstream impeller/nozzle section maybe also configured to operate at pressures above 276 kPA.

In energy terms this means that the downstream impeller blades, in thesenew designs, impart kinetic energy directly to the jet stream. Furtheradvantages include the removal of back pressure effects on thedownstream impeller and losses arising from pressure energy conversionat the nozzle outlet. In these designs the nozzle is now placed betweenthe impellers and the opening, downstream of the downstream impeller, isnow merely an outlet for the pump (As opposed to being a nozzle). Theintroduction of air to the downstream impeller. FIGS. 2, 3, 4, 5 and 6further reduces frictional losses in the impeller casing but moreimportantly allows the downstream impeller to operate at atmosphericpressure.

MODE OF OPERATION

To facilitate priming of the upstream impeller 5 an external pressurecontrol priming device 10, comprising a collapsible skirt and peripheralspring, as seen in FIGS. 1, 2, 5 and 6 is placed between the twoimpellers 5 and 6. In FIG. 3 the pressure control priming device 19 isfixed to the centre of the upstream impeller 5 and consists of acollapsible skirt 20 within which is placed a plunger cone 21 andtensioning spring (not shown) whereby the pressure of the water forcesthe skirt 20 and plunger cone 21 in and out. The air inlet(s) 12 in thepump casing, FIGS. 2, 3, 4, 5 and 6 are provided with close-off flaps13, FIG. 2 which are pressure controlled. Once a primed condition isachieved they remain open to permit continuous air entry. The provisionof air inlets 12 thus allows the downstream impeller 6 to assist inpriming the pump when they are closed. In FIG. 1 no air entry ispermitted between the two impellers, but delivery rates between the twoimpellers must be carefully adjusted to ensure that the two impellersare hydraulically balanced in respect of flow rate, so that thedownstream impeller always operates at atmospheric pressure.

A further improvement allows for the blades of the downstream impeller 6to be automatically adjusted whereby the peripheral blade angles of theimpeller 6 may be varied or calibrated according to the helical flowimpinging on it from the upstream impeller 5. This feature is madepossible because the blade to pump housing clearances are much greaterthan that required of a pressure pump so that the blades of the impeller6 may be rotated slightly within the circular casing of the pump housing8.

In very simple terms, the devices described are thus a pressurised pumpsection, containing the upstream impeller, followed by a propelleroperating at atmospheric pressure, enclosed in a casing.

In a further design departure, not shown, the downstream section of theunit may simply consist of a ringed impeller whereby a ring is fixeddirectly to the outer edge of the impeller blades. No pump casing thusbeing required.

Also not described is a pump of essentially the same designconfiguration and having the same operating criteria, as described inany of the drawings, whereby the upstream impeller is of “mixed flowdesign”, followed by a downstream impeller of “axial flow” design. Inthis case the pressure control priming device is also between theimpellers, together with the features already outlined for the totallyaxial flow design (FIGS. 1 to 6).

The designs are based on the principle of a high mass, low pressure andthrottled configuration as described in our NZ patent 256488, such thatimproved efficiency is achieved by maximising the flow rate through thejet propulsion unit at the lowest possible internal unit pressure.Typically, impeller peripheral blade angles fall in the range of about30 to 50 degrees, depending on power input but may fall outside thisrange should impeller diameters be altered or the pumps operatingrequirements change. Impeller peripheral tip speeds, relative to in-pumpflow velocities, are usually limited to the range of about 45 to 65meters/second, to restrict the damaging effects of cavitation. Forspecific applications, for example boat racing, where high boat speed isrequired, such a peripheral tip speed restriction may, however, beignored by the user. The provision of air to the downstream impelleralso helps to reduce the effects of cavitation. In respect of impellerdesign, the downstream impeller is no longer required to have a“pressure” configuration where the blades are normally aligned orover-lapped. Instead the blades may have a more open architecture asapplies in conventional propeller design or a “cleaver” shape typical ofthose found in surface piercing drives. In our case however it isdesirable to maintain the outer edge of the blade so that a largeportion is contiguous with the wall of the pump housing in order tobetter control the amount of work carried out by the blade.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 describes a basic pump unit in a simplified side elevational viewwith no specific facility for air to be introduced between theimpellers. Air may, however, pass down the centre of the jet plume thusallowing some control over pressure between the impellers. A pressurecontrol priming device is located between the impellers at the peripheryof the pump casing.

FIG. 2 describes a basic pump unit in a simplified side elevational viewwith an air entry control system located between the impellers. Apressure control priming device is located between the impellers at theperiphery of the pump casing.

FIG. 3 describes a basic pump unit in a simplified side elevational viewwith an air entry control system located between the impellers. Apressure control priming device is located between the impellers but inthis example is fixed to the centre of the pump casing.

FIG. 4 is an external detailed three dimensional view of the pump unitshowing the air entry control system, shown also as FIG. 2.

FIG. 5 is cutaway view of FIG. 4 showing the air entry control system,impellers and pressure control priming device (a spring loaded skirt)inside the pump casing.

FIG. 6 is a part cutaway view of FIG. 4 showing the air entry controlsystem, the pressure control priming device (a spring loaded skirt) andthe two impellers.

DESCRIPTION OF PREFERRED EMBODIMENT

FIGS. 4, 5 and 6 describe an axial flow water jet propulsion unit wherean engine may be directly coupled to the transmission 1.

In FIG. 5, (External view FIG. 4 without cut away section) thepropulsion unit consists of a transmission 1 providing counter-rotationof coaxial shafts 2 and 3. The design details for this are outlined inour New Zealand Patent 256488. Water is drawn through an intake section4 thence through impellers 5 and 6 contained within pump housings 7 and8 which are fixed to coaxial shafts 2 and 3. A nozzle section 9 islocated between the two impellers 5 and 6 and includes a spring loadedcollapsible skirt 10 which helps to facilitate priming and control ofpressure inside the unit. The upstream impeller 5 and nozzle section 9are pressurized in the range of about 0-276 kPa (or greater if desired)over the operating range of the unit. The helically spinning waterpasses through the nozzle section 9 and impinges on the downstreamimpeller 6 which is operating at atmospheric pressure. Air 11 enters thearea immediately downstream of the nozzle section 9 thus reducingcavitation and friction but primarily serving to limit hydraulic suctionon the downstream impeller 6, thereby maintaining a constant operatingenvironment approximating atmospheric pressure. To assist priming andmaintain a low pressure environment the air inlets 12 are controlled bya sliding ring 13, FIG. 5 which is operated by a hydraulic spring loadedslave cylinder 14, FIG. 4 which pushes the ring backwards/forwards. Inan alternative arrangement, as seen in FIG. 2, flaps 15 are used forthis purpose. The flaps 15 being controlled either hydraulically, by thepressure of the water or indirectly by electro-magnetically controlledlatches, not shown, so that they are closed at start-up and fully openat full power. A three or four vane bearinged support 16 in the outletto the pump housing 8 provides support for the coaxial drive shafts 2and 3. Steering flaps 17 are attached to the pump housing 8 by pinnedhinges 18. A steering mechanism is not shown.

What is claimed is:
 1. A water jet propulsion unit comprising: an intakesection; a pump section which includes a pump housing enclosing anupstream axial flow impeller followed immediately downstream by apressure control priming device; followed by a second pump housingcontaining a further downstream impeller of opposite pitch; saiddownstream pump housing having an air entry control system; said airentry control system including a sliding ring being hydraulicallyoperated so that the quantity of air entering the downstream pumphousing can be controlled precisely; said air being permitted to enterthe downstream pump housing via ports cast into the downstream pumphousing; said impellers being separately mounted on contra rotatingshafts; said coaxially arranged shafts being located at the outlet endof the downstream pump housing by a bearinged three or four vanesupport; said impellers being able to individually rotate at peripheralspeeds above 30 meter per second; said downstream impeller beingpermitted to operate at atmospheric pressure whilst in operating mode;said intake housing, upstream and downstream pump housings, all being insmooth communication with each other; said water jet propulsion unitbeing able to be configured to operate in a pressure range above orbelow 276 kPA.
 2. A propulsion unit as claimed in claim 1, wherein theimpellers have peripheral blade angles in the range of 10 to 50 degreesdepending on pump configuration.
 3. A propulsion unit as claimed inclaim 1, wherein the downstream impeller has a ring attached to itsblades such that no enclosing pump housing is required.
 4. A propulsionunit as claimed in claim 1, wherein the pressure control priming deviceis located between the two contra rotating impellers.
 5. A propulsionunit as claimed in claim 1, wherein the pressure control priming deviceis mounted to the centre of the upstream impeller.
 6. A propulsion unitas claimed in claim 1, wherein the entry of air is controlled by flapsbeing able to be activated remotely by electro mechanical means.
 7. Awater jet propulsion unit comprising: an intake section; a pump sectionwhich includes a pump housing enclosing an upstream impeller followedimmediately downstream by a pressure control priming device; followed bya second pump housing containing a further downstream impeller ofopposite pitch; said downstream pump housing having an air entry controlsystem; said air entry control system including a sliding ring beinghydraulically operated so that the quantity of air entering thedownstream pump housing can be controlled precisely; said air beingpermitted to enter the downstream pump housing via ports cast into thedownstream pump housing; said impellers being separately mounted oncontra rotating shafts; said downstream impeller being permitted tooperate at atmospheric pressure whilst in operating mode; said intakehousing, upstream and downstream pump housings; all being in smoothcommunication with each other.